Conventionally, a voltage nonlinear resistor, which is used for a lightning arrester, a surge absorber, or the like, is made of a sintered body, which is produced by a process including crushing, mixing, granulating, forming, firing, and post heat treatment of a composition containing zinc oxide (ZnO) as a main ingredient, bismuth oxide that is essential for expressing voltage nonlinearity, and an additive that is effective in improving electric characteristics. This sintered body is provided with electrodes and a side surface high-resistivity layer so as to constitute the voltage nonlinear resistor.
Action of a voltage nonlinear resistor is roughly divided into a stand-by state in which no surge energy is applied and an operating state in which surge energy is applied. Currently, voltage nonlinear resistors are mainly used in gap-less structures in which voltage is always applied across both ends in the stand-by state. Therefore, it is important that current flowing in an element in the stand-by state (leakage current) does not show a tendency to increase. In order that the leakage current does not show a tendency to increase, that is, in order to secure good loading service life characteristics, heat treatment after firing is usually essential (see, for example, Patent Literature 1 and 2). This heat treatment after firing prevents the leakage current from showing a tendency to increase, and hence prevents thermal runaway due to an increase in heat value of the voltage nonlinear resistor accompanying an increase in leakage current can be prevented. However, if heat treatment is performed after firing, the voltage nonlinearity of the voltage nonlinear resistor is usually prone to large deterioration. In order to prevent this, there is also proposed a method involving performing the heat treatment in two steps (see, for example, Patent Literature 3).
As an index indicating whether the voltage nonlinearity is good or bad, a flatness ratio is used. The flatness ratio is defined to be the ratio between voltages generated across both ends of the voltage nonlinear resistor when two currents having different values are supplied to the voltage nonlinear resistor, and the values of currents used for the evaluation depend on the diameter of the voltage nonlinear resistor. For example, as the flatness ratio, there is used a ratio (V10kA/V2mA) between a voltage value (V10kA) when 10 kA current is supplied which corresponds to large current region characteristics and a voltage value (V2mA) when 2 mA current is supplied. Efforts are being made to develop a technology for improving the voltage nonlinearity of voltage nonlinear resistors, in other words, to reduce the flatness ratio.
The performance of voltage nonlinear resistors in the stand-by state and the operating state described above depends largely on the fine structure of the sintered body. The sintered body generally includes zinc oxide grains, spinel grains containing zinc and antimony as main ingredients, and bismuth oxide phases existing in the vicinity of grain boundary triple junctions. Other than that, as an additive, there are observed zinc silicate grains containing silicon as a main ingredient. Bismuth, which is an additive essential for expressing voltage nonlinearity, is well known to exist not only in a bismuth oxide phase but also in very little amounts in the grain boundaries between zinc oxide grains (see, for example, Non-Patent Literature 1). Efforts are being made to resolve the structure thereof and to measure the interface level of the grain boundary.
In recent years, processes for obtaining voltage nonlinear resistors having good voltage nonlinearity at low cost, by decreasing the firing temperature of the voltage nonlinear resistor to 1,000° C. or lower have been disclosed (see, for example, Patent Literature 4). It is known that an appropriate Sb2O3/Bi2O3 ratio must be selected in order to reduce voids in the sintered body that deteriorate the voltage nonlinearity of a voltage nonlinear resistor and the break-down threshold value (withstand energy) when large amount of energy is applied to a voltage nonlinear resistor, so that a fine sintered body can be obtained, by firing at 1,000° C. or lower (see, for example, Non-Patent Literature 2). Non-Patent Literature 2 describes that the Sb2O3/Bi2O3 ratio is set to 0.5 as an example, and that rapid densification occurs at a firing temperature of 900° C. Further, evaporation of bismuth oxide in the firing process is one factor generating voids. However, when the firing is performed at a relatively low temperature of 1,000° C. or lower, evaporation of bismuth oxide in the firing process can be substantially suppressed. The synergistic effect of the suppression of void generation and the densification enables the voltage nonlinearity and the withstand energy of the voltage nonlinear resistor to be improved. In other words, in a firing process at 1,000° C. or lower, the Sb2O3/Bi2O3 ratio is a parameter that largely affects the densification and the voltage nonlinearity of a voltage nonlinear resistor.
In this way, by firing at 1,000° C. or lower, a voltage nonlinear resistor having good voltage nonlinearity can be obtained at low cost. However, in recent years, voltage nonlinear resistors having better voltage nonlinearity and loading service life characteristics are in demand.    [Patent Literature 1] Japanese Patent Laid-Open No. 52-53295    [Patent Literature 2] Japanese Patent Laid-Open No. 50-131094    [Patent Literature 3] Japanese Patent Laid-Open No. 58-200508    [Patent Literature 4] Japanese Patent Laid-Open No. 2003-297612    [Non Patent Literature 1] Kei-Iciro Kobayashi, Journal of American Ceramic Society, “Continuous Existence of Bismuth at Grain Boundaries of Zinc Oxide Varistor without Integranular Phase”, 81, [8], 2071-2076 (1998)    [Non Patent Literature 2] Jinho Kim, Toshio Kimura, and Takashi Yamaguchi, Journal of American Ceramic Society, “Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide”, 72, [8], 1390-1395 (1989)